The thermo-fluid reacting environment and local thermodynamic state in solid oxide fuel cell (SOFC) stacks were examined by using three-dimensional numerical simulations. Enhancing the performance and durability of the SOFC stacks is essential when a high fuel utilization scheme is implemented to increase the system efficiency and lower system operating costs. In this study, numerical simulations were conducted to elucidate the effect of fuel utilization on heat and mass transfer as the fuel utilization is raised. A high-fidelity three-dimensional physical model was developed incorporating elementary electrochemical reaction kinetics by assuming rate-limiting steps and spatially-resolved conservation equations. The model considers planar anode-supported SOFC stacks and is validated against their electrochemical performance experimentally measured. A parametric study with respect to fuel utilization was conducted by varying a fuel flow rate while maintaining other operating conditions constant. Results show that, when increasing the fuel utilization, a narrow and non-uniform electrochemical reaction zone is observed near the fuel inlet, resulting in substantial depletion of hydrogen in the downstream fuel flow and thus raising the partial pressure of oxygen in the anode. This subsequently lowers the electrochemical potential gradient across the electrolyte and hence induces a large gradient of ionic current density along the cell. Convective flow through porous electrodes also results in pressure gradients in the direction of both cell thickness and length. In addition, the heat balance between conduction through metallic interconnects, convection by gases and the heat generated from charged-species transport and electrochemical reactions determines a temperature gradient along the cell and its maximum location. All of these gradients may induce chemical, mechanical and thermal stresses on SOFC materials and correspon